The Assad regime on Sunday attacked the besieged Damascus suburb of Eastern Ghouta with chlorine gas, according to the White Helmets, a pro-opposition Syrian civil defense agency.
One child was killed in the attack which targeted the Al-Shifoniya town, the agency said on its official Twitter account, adding widespread suffocation occurred among civilians including two Syrian Civil Defence volunteers.
Duma Hospital officials confirmed to Anadolu Agency that 16 people had been poisoned in the chemical attack.
The regime forces have attacked Duma thrice with chlorine gas in the last two months.
Earlier on Sunday, regime forces also mounted air and artillery attacks in the towns of Duma, Harasta, Al-Shifoniya, Kafr Batna, Saqba, Beit Sawa and Al-Marj, said a White Helmets source who spoke on condition of anonymity for security reasons.
Residential areas in the city of Jisr al-Shughur in the northwestern Idlib province were also reported to have come under heavy shelling by regime forces.
Regime warplanes also struck the town of Kafr Zita in the central Hama province, the source said.
The death toll from the attacks rose to five, including a woman and a child.
The UN Security Council on Saturday adopted a resolution calling for a 30-day cease-fire in Syria to allow for humanitarian aid deliveries.
The cease-fire decision came as regime forces intensified attacks on Eastern Ghouta in recent days, killing several hundred people.
Home to some 400,000 residents, Eastern Ghouta falls within a network of de-escalation zones -- endorsed by Turkey, Russia and Iran -- in which acts of aggression are expressly prohibited.
The resolution, prepared by Sweden and Kuwait, also calls for the medical evacuation of 700 people, particularly in Eastern Ghouta, which has been under siege for the last five years, preventing the delivery of food and medicine and leaving thousands of patients in need of treatment.
Syria has been locked in a devastating civil war since early 2011 when the regime cracked down on demonstrators with unexpected ferocity.
According to UN officials, hundreds of thousands of people have been killed in the conflict to date.